Design of multistage extractive reaction processes

Multistage extractive reaction (MSER) processes can successfully improve the yield of and selectivity to the desired product, as well as separation of byproducts. A general and efficient design procedure for stand-alone countercurrent MSER cascades and process flowsheets presented here determines the range of feasible operation for the MSER process, the number of stages in the MSER cascade, compositions and molar flow rates at each stage, and the yield or product distribution of the reaction system. It is based on the geometric properties of the phase and reaction equilibrium surfaces and the composition profiles of the MSER cascade. First, a key design variable is identified and the fixed points or pinches of the composition profiles are tracked as a function of this variable by using arc-length continuation, without performing stage-to-stage calculations. The location of the fixed points gives the upper or lower bound on the window of the MSER operation. The other bound is determined by the equilibrium behavior of the system. Within the window of operation MSER profiles are guaranteed to be feasible. Then, stage-to-stage calculations are performed as an initial-value problem to complete the design. The results of these calculations provide a comprehensive picture of the trade-offs in the design of MSER processes and can be used as a basis for screening flowsheet alternatives. The procedure is illustrated with several design examples.